(1) Field of the Invention
The present invention relates generally to an apparatus for measuring the linear displacement of a reciprocating member and, more specifically, to a spring force loaded contact wheel device for measuring transient displacement of a linear shaft having a smooth surface.
(2) Description of Prior Art
Devices for measuring the linear displacement of a reciprocating shaft are useful in providing data for parametric testing of a Ram Pump Ejection System (RPES) used in launching torpedoes or other devices from a submarine or other marine vessel. A typical RPES includes a shaft having a piston mounted at each end thereof, each piston being housed in a separate cylinder wherein shaft seals maintain an isolated environment for each cylinder as the shaft travels laterally therethrough. In operation, a first piston of the RPES is pneumatically driven through its stroke by a pulse of high pressure air, creating a force against the connecting shaft, which, in turn, drives the second piston through its stroke. The second piston is located in a hydraulic environment such that the stroke thereof generates a hydraulic impulse for launching the weapon or other device. In the RPES described hereinabove, the pistons and the associated connecting shaft travel over a fixed stroke in a transient manner. Each piston is located in a limited access environment such that measurement of the displacement thereof is limited to the environment outside of each cylinder along the exposed portion of the shaft. The method presently employed for collecting linear displacement data in an RPES involves the application of coded information directly onto the shaft for detection by an optical sensing device. However, it has been found that this coded information tends to wear off quickly as the shaft travels through the shaft seals and also gives rise to undue wear on the shaft seals such that periodic maintenance, requiring time consuming disassembly of the RPES is required. Further, the present method used for collecting displacement data cannot be readily applied to shipboard system evaluations and is therefore limited in its application to use at launch system test facilities such as one that simulates an RPES on a 688 class submarine. The present invention contemplates the use of a contact wheel and associated rotary encoding device to measure linear displacement of a shaft. Since an RPES involves a shaft connecting two pistons which are necessarily located in pressure sensitive cylinders, the connecting shaft between the two pistons must translate through multiple sets of seals, thereby necessitating that the shaft have a very smooth surface. Further, since at least one end of the shaft travels through a hydraulic environment, the shaft is often wet and slippery. These restrictions tend to create difficulty in maintaining absolute non-slip, friction contact between a contact wheel and the reciprocating member which can yield erroneous displacement measurements. The present invention provides a novel friction drive apparatus for measuring linear displacement of a smooth reciprocating shaft.